In Time Division Multiple Access (TDMA) communication systems, subscriber units must initially access the communication system at a proper frequency and time slot. Failure to do so will result in the inability for the subscriber to access the system and interference with ongoing calls in adjacent time slots. This is especially true where the timing windows for acquisition to communication stations are short and the frequency bandwidth is narrow. In satellite based communication systems where the communication stations are moving with respect to the subscriber units, subscriber units must compensate for the propagation delay associated with the distance to a satellite and the frequency shift associated with the velocity of the satellite. This is difficult to do without knowing the distance to the satellite and the satellite's velocity with respect to the subscriber unit. This situation is even more difficult where the satellites project multiple beams and the size of the beams is very large because difference in the propagation delay across a beam may exceed the satellite's acquisition timing window. This is especially true in low-Earth orbit satellite systems because the positions and orbits of the satellites are not fixed with respect to the surface of the Earth.
Thus what is needed is a method and apparatus for accessing a communication system that projects antenna beams that are hundreds or even thousands of miles in length. What is also needed is a method and apparatus for synchronizing to a communication system's timing windows where the distances to the subscriber units are unknown. Thus what is also needed is a subscriber unit that determines its position relative to a communication station and sends acquisition signals in sync with the system's acquisition timing windows. Thus what is also needed is a method and apparatus of initially accessing a Time Division Multiple Access (TDMA) communication system without interfering with ongoing calls in adjacent time slots and adjacent frequency channels.